This Core focuses on antidepressant studies and is particularly concerned with the assay of nortriptyline and paroxetine platelet 5-HT reuptake interrupted by the use of antidepressant medication, including a double- blind study of paroxetine, a potent innovative serotonin reuptake inhibitor versus the more generally used nortriptyline. The goal of the study is to document pharmacokinetic changes in elderly adults. one primary goal of the present Core is to provide support for other cores and relevant projects of the investigators focused on pharmacokinetic/pharmacodynamic assessments, and to provide relevant instruction for students and area nursing homes, in vivo (phenotyping) and in vitro (drug metabolic assays) studies of cytochrome P450 function, an enzyme responsible for metabolism of significant antidepressants.